1. Field of the Invention
The present invention relates to a technique to improve sensitivity of an imaging apparatus.
2. Description of the Related Art
Conventionally, regarding a video signal of an imaging apparatus, light passing through a lens is formed as an image on an image sensor via an infrared ray cut filter (hereinafter, referred to as IRCF). The IRCF prevents the infrared rays, which is invisible to the human eyes, from entering the image sensor and prevents a video from becoming reddish by the infrared rays. Then, the imaging apparatus converts the light whose infrared rays are cut into a video signal by the image sensor, executes white balance (hereinafter, referred to as WB) adjustment from the video signal to balances red, green and blue (RGB) components, and outputs the adjusted signal as a color image.
In addition, the imaging apparatus extracts the IRCF located in the previous stage of the image sensor from the optical path and causes the image sensor to receive the infrared rays cut by the IRCF to improve sensitivity. However, extraction of the IRCF may disturb the RGB balance of the video signal, and thus, it becomes difficult to achieve the WB. Accordingly, the imaging apparatus uses a technique for changing from output of a color image to output of a monochrome image when the IRCF is extracted. Such the technique enables the imaging apparatus to capture an image which is the monochrome image but has improved sensitivity. Further, by such the technique, the imaging apparatus can increase the luminance of the image and perform shooting of a low-illuminance object with excellent visibility.
Conventionally, various techniques for improving the sensitivity have been considered. As a technique to brighten a darkened image regardless of insertion and extraction of the IRCF, the following technique is known. In a technique discussed in Japanese Patent Application Laid-Open No. 2009-159017, a luminance value of an image before and after the WB adjustment is memorized, and the difference therebetween is added to the image after WB adjustment to solve a reduction in luminance due to the WB adjustment.
However, the technique discussed in Japanese Patent Application Laid-Open No. 2009-159017 only corrects the reduction in luminance caused by executing the WB adjustment and cannot improve the sensitivity more than that. Further, the conventional method for improving sensitivity by the insertion and extraction operation of the IRCF requires time to move the optical filter accompanied by the insertion and extraction operation, and is inferior in responsiveness when a mode is shifted to the one of improving sensitivity. Furthermore, since a holding frame of the IRCF intersects the image sensor when the IRCF is inserted and extracted, an image is hard to see when the IRCF is inserted and extracted.